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Challenge mode anon here. Defeated Cheren!
Seems like all I needed was to catch a bug. Almost fainted 2/3 before I had to switch out. I feel like I could've brought him in sooner and still done okay. Didn't even need to grind half my team. He did way better than expected. His stats are higher than my starter and probably the rest of my team, and bug bite helped to nerf Lillipup and its oran berry.
Looking into the numbers, apparently it's the mon with the highest encounter rate so I don't know why it took so long to find.
I think on my Black playthrough it ended up being replaced by Scolipede but I'm looking to use different mon this time, and I think this one's staying.